Contractile protein interactions in smooth muscle.

Smooth muscle tone and 'holding economy' depend on the rate constants governing the cross-bridge cycle. Thus, calcium activation via calmodulin-dependent myosin light chain phosphorylation may determine the apparent rate constant ('f') at which cross-bridges enter the force-generating state, forming actin-attached, strongly bound cross-bridges. This phosphorylation of the light chain may be inhibited in skinned fibers by a peptide mimic of the calmodulin recognition site of the myosin light chain kinase (RS 20) that relaxes smooth muscle. In smooth muscle, the apparent cross-bridge detachment rate constant ('g') also seems to be variable, a low constant allowing for a high holding economy and low shortening velocity in the 'latch state'. It may also account for force maintenance at low levels of myosin phosphorylation. Additionally, cross-bridge attachment may, however, be also controlled by other regulatory proteins such as calponin and caldesmon.